Modulator swapping - should I?

Discussions about Sinclair ZX80 and ZX81 Hardware
SabreWulf
Posts: 19
Joined: Mon Jul 26, 2021 6:31 pm

Modulator swapping - should I?

Post by SabreWulf »

One ZX81 became two. How did that happen? I'd better watch out as there's at least six Spectrums. Don't feed them after midnight, don't let them near water, don't tell them your eekbay password.

ZX81 1: socketed ICs, non back-porch ULA, original modulator, 622 ROM.

ZX81 2: soldered ICs, back-porch ULA, gutted modulator with a zx.zigg.net type (Rudedog made) 555 added back-porch composite output, 622 ROM.

I have colour CRT TVs and some composite input monitors and there's a lingering nostalgic insanity in having at least one ZX81 with with a usable RF display.

1 produces a typical non back-porch RF display.

2 produces a reasonably sharp and bright, but horizontally trembling at the right hand edge, display on composite. I was expecting something odd with the ULA and 555 combination but wasn't sure what it would look like. Is the modification adding to and/or distorting the back-porch? If only I'd decided on an oscilloscope, I'd be able to see it.

As tape input and output is also on pin 16, do any of the composite modifications affect loading and saving? It's a passing thought as I'm in the midst of unearthing and resurrecting several old cassette recorders and I've so far failed to successfully load and save. It's most likely due to non ZX81 factors but it's best to ask.

I think that the optimum solution is to swap the modulator circuits between 1 and 2 to produce two functional displays: one RF, one composite and both with back-porches. If the ULA on 2 had been socketed, I'd have carefully swapped ULAs, but in the days before a desoldering station I have distant traumatic memories and survivor's guilt in trying and largely failing to cleanly remove a functioning soldered ULA from a Spectrum.

The modulator appears to have two substantial gnd mounting solder points with easily de-solderable (famous last words) 5v and video connections. Are the gnd mounts challenging to desolder? Is there anything of interest underneath the modulator casing that I can't yet see? Some Spectrums had one or both of the modulator mounts folder over before they were soldered and that made them a little more difficult to desolder.
bola_dor
Posts: 398
Joined: Wed Oct 02, 2019 5:32 am

Re: Modulator swapping - should I?

Post by bola_dor »

I think you have to remove that 555 mod from the 2nd zeddy.
I am not sure but I ve seen somewhere else that taking the IC out from the board will keep the buffering without the backporchand then you will have a good composite output .
Or use the buffer composite mod on MAK1024 signature.. I've used it a couple of times and works very good.
I tried once the 555 mod, made my self, on a newer ULA and got a thick black border to the left. So I guess that disturbance you see has to be with the "double" backporch.
As for the 1st Zeddy. Is the output at tour CRT good enough? Then keep it like that if you are getting another good composite output in your second..
Or make simple transistor/resistor mod with a 1N4148 diodes at the ULA output (this not only protects your ULA but makes newer TVs to like more the signal and keeps it easily in Sync. Mount it with tiny clamps and just take the output with a cable beside the original video out. You don't have to disable the RF modulator for this to work. You need to disable with Mark's mod but it's much better
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
SabreWulf
Posts: 19
Joined: Mon Jul 26, 2021 6:31 pm

Re: Modulator swapping - should I?

Post by SabreWulf »

Hello Ernesto,

Thanks for your thoughts - they help me make a decision.

The RF output of 1 isn't good enough; it's woefully dark on my colour TVs. The 555 on 2 isn't socketed and it will need an amount of dismantling just to get to it. I purposely sought out another ZX81 with a later ULA to see what the RF output was like - knowledge seeking really - and the one that I found happened to have the 555 mod fitted, with the seller (unsurprisingly) describing an unstable picture... It sounded interesting as it gave me two things that I was curious about and so I made a low offer. For the moment, I like the simplicity in using what I've already got and that's why I'm pondering out loud about swapping the modulator circuits in their entirety.

It's highly likely that I'll add a ZX8-CCB, out of curiosity, in the future; should it become available and I'm not trampled in the rush. There's also no doubt that I'll build a 555 circuit, and an external RAM-pack using Grant Searle's elegant design. Why external? Why not?
bola_dor
Posts: 398
Joined: Wed Oct 02, 2019 5:32 am

Re: Modulator swapping - should I?

Post by bola_dor »

Shure!!!
I didn't have good results with the 555 board. Perhaps I wasn't lucky with the IC I got.
There's a long thread about 555 IC and boards this year somewhere...
I have my ZX80 connected the way I tellyou buy I guess my TV is more tolerant. Some times I have to blindly execute a SAVE at start up to get it on sync.. :mrgreen:
Good luck desoldering the RF boxes.. I wouldn't attempt to empty it (to keep the esthetics ) as it probably the board won't get out easily in one piece. RF modulator board is soldered to the box inside and with a really hard solder.. you have to remove the modulator from the main board anyway
Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
bola_dor
Posts: 398
Joined: Wed Oct 02, 2019 5:32 am

Re: Modulator swapping - should I?

Post by bola_dor »

Ernesto
ZX80 USA, ZX81UK, ZX Spectrum, ZX Spectrum+, ZX Spectrum 128+ UK, ZX Spectrum +2/A, Sinclair QL, CZ1000, CZ1500, CZ2000, CZ1000Plus, CZ1500Plus, CZ Spectrum, CZ Spectrum Plus, TK83, TK85, TK90X, TK95. TS2068. And more to come :D
Lardo Boffin
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Joined: Sat Nov 26, 2016 2:42 am

Re: Modulator swapping - should I?

Post by Lardo Boffin »

SabreWulf wrote: Sat Sep 04, 2021 1:18 pm One ZX81 became two. How did that happen? I'd better watch out as there's at least six Spectrums. Don't feed them after midnight, don't let them near water, don't tell them your eekbay password.
2? Those are rookie numbers! :D

75AD559B-F3E8-483D-BE43-2CC65799A3D3.jpeg
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
SabreWulf
Posts: 19
Joined: Mon Jul 26, 2021 6:31 pm

Re: Modulator swapping - should I?

Post by SabreWulf »

I'm doing my best to keep it at two.

Once upon a time, and not that long ago, I formulated a plan to acquire a working Spectrum. I'd write better machine code than the first time around and learn something new. But that rubber-keyed marvel had faulty upper RAM and I'd always wanted to better understand the hardware and I enjoy fixing things, and I'd had a Plus long ago and no one else was interested on eekbay, and then I remembered how much I'd wanted a 128K in 1986, and how I'd had an Oric and missed it, and how great the thumping tunes were on my CBM 64 when I crossed to the dark side, but that I no longer owned... And I'd never had a Beeb and the inbuilt assembler had always intrigued me and I wanted to see how the other half had lived... But, I'm better now. The recent eekbay activity? I can handle it, I can. I can, can't I? Besides, I'm enjoying the repairs and the thinking about them. It's the RAM pack dreams that are a bit of a worry. Now, what about that oscilloscope? How hard is it to desolder the modulator?

It's gremlins, it must be. I can hear them.
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: Modulator swapping - should I?

Post by Moggy »

Place iron next to a modulator tab, when the solder is runny suck up excess with solder pump.
Place a very thin bladed screwdriver or knife between mod' and c/board, apply heat to one of the mod' lugs and twist to release lug then hold the mod by hand and do the same for the other lug taking note of the two wires going into the mod'. These can be un-soldered just as easily.

The mod' is a very hardy device so stop worrying about damage etc.

Once un-soldered it's up to you what you do with it, personally I remove the board destroying it if need be as I have no reverence for originality or interest in going back to UHF and anything that makes it more useful is ok for me though other opinions may differ.

If the ZX8-CCB becomes available get one because as well as composite you can have switchable inverse with full black border too.
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: Modulator swapping - should I?

Post by Lardo Boffin »

SabreWulf wrote: Sat Sep 04, 2021 5:52 pm Once upon a time, and not that long ago, I formulated a plan to acquire a working Spectrum. I'd write better machine code than the first time around and learn something new. But that rubber-keyed marvel had faulty upper RAM and I'd always wanted to better understand the hardware and I enjoy fixing things, and I'd had a Plus long ago and no one else was interested on eekbay, and then I remembered how much I'd wanted a 128K in 1986, and how I'd had an Oric and missed it, and how great the thumping tunes were on my CBM 64 when I crossed to the dark side, but that I no longer owned... And I'd never had a Beeb and the inbuilt assembler had always intrigued me and I wanted to see how the other half had lived... But, I'm better now. The recent eekbay activity? I can handle it, I can. I can, can't I?
You must have the same voices as me! My relatively recent madness started with ‘wouldn’t it be nice to get a Beeb again and final learn machine code?’

Well, I have a Beeb (or 5) and a part time job refurbishing zeddies and guess what? Still not learned 6502 machine code. Although I have a lot of books… And many other retro classics I really wanted BITD.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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1024MAK
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Re: Modulator swapping - should I?

Post by 1024MAK »

The link to the first post in the NE555 back porch saga is here ;-)

If it’s hard to get to the NE555 back porch board that has been fitted into the modulator case and you want to keep it, then ‘swapping’ modulators may appear easier. However, the metal case means you do need a powerful soldering iron with a big chisel tip (I use a 50W iron with a 5mm chisel tip). As the metal modulator case sucks the heat away from the solder joint…

Does the NE555 back porch board have one, two or no preset (adjustable) resistors?

Mark

PS you can ask how many 8 and 16/32 bit computers I have, just don’t expect an answer. Mainly because (a) I’ve lost count and (b) if I knew how many I’ve got, I’d have to admit, err, well, I’m sure you get the idea :shock:

I have listed most of the makes/models that I have in a topic on one of the forums that I take part in, but can’t remember which one.
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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